1. Field of the Invention
This invention relates to an optical illumination device for efficiently introducing light emitted from a light source into a light transmitting fiber.
2. Description of Related Art
FIG. 1 shows an optical illumination device which has been used in the past. This optical illumination device is such that most of light originating from a light source 1, with the exception of a fraction of light incident directly on a light transmitting fiber 3, is reflected and condensed by an elliptical mirror 2 and is incident on an entrance end 3a of the light transmitting fiber 3 located at the position of a focal point. Instead of the elliptical mirror 2, a concave mirror which is constructed with, for example, a parabolic, spherical, conical, or high-order curved surface has also been used.
The optical illumination device, when used in an endoscope, requires a considerable brightness to observe the interior of a dark cavity of the human body. Although the need for an endoscope with a small diameter is emphasized in particular, the brightness of the light source is generally not satisfactory for such a fine endoscope, and even from this viewpoint, an improvement of the brightness has been desired.
Consequently, the optical illumination device utilized for endoscopes requires rays of light emitted from the light source to be rendered as copious as possible (namely, a high condensation efficiency) and to be condensed in the smallest possible area (namely, a small condensation diameter).
In order to improve the brightness of the light source, where a xenon lamp which is short in light emission length and high in luminance is used as the light source, optical illumination devices having practical use have been proposed in the past. Here, the term "light emission length" means a dimension in the longitudinal direction of the light source which has a depth in a space of, for example, a light emitting region formed between the anode and the cathode of an electric discharge lamp or a light emitting section of a filament lamp.
For example, an optical illumination device such as that shown in FIG. 2 is available in which a reflecting mirror 4 is placed opposite to the elliptical mirror 2 between the light source 1 and the light transmitting fiber 3 so that light from the light source 1 which will be diffused between an opening edge 2a of the elliptical mirror 2 and the entrance end 3a of the light transmitting fiber 3 is reflected back to the light source 1, with a resulting increase in condensation efficiency. Alternatively, as shown in FIG. 3, an optical illumination device is available in which a condenser lens 5 is interposed between the light source 1 and the light transmitting fiber 3 to reduce the magnification of projection of the light source 1 on the entrance end 3a of the light transmitting fiber 3, thereby raising the condensation efficiency.
The xenon lamp of short light emission length and high luminance, however, is expensive in itself and generally requires large power consumption. Furthermore, a power supply for turning on the lamp and cooler are also expensive and oversized, and thus the problem is encountered that the optical illumination device using the xenon lamp is costly and becomes bulky. In contrast to this, where a metal halide lamp or a halogen lamp in which the light emission length is as long as 4-7 mm, but the lamp itself is lower in cost and requires less power to operate, is used in the optical illumination device, the power supply for turning on the lamp and the cooler are constructed at low cost and from compact design, and hence an inexpensive, small-sized optical illumination device can be provided. For this reason, the metal halide lamp and the halogen lamp are now being reconsidered.
Each of the optical illumination devices proposed in the past, mentioned above, because the light emission length of the light source is short, is effective for the use of the xenon lamp which has a relatively small bright-spot diameter and approximates a point source (and also has high luminance). Such an optical illumination device, however, is not effective for the case where the light emission length of the light source is longer, from the following reasons.
A case where the light source is a point source is first considered, As shown in FIG. 1, all rays emitted from the point source 1 lying at a first focal point F1 of the elliptical mirror 2 and reflected by the elliptical mirror 2 are condensed at a second focal point F2. As depicted in FIG. 2, when the reflecting mirror 4 having the center of curvature at the first focal point F1 of the elliptical mirror 2 is placed opposite to the elliptical mirror 2, rays emitted from the light source 1 and reflected by the reflecting mirror 4, after being condensed at the position of the light source 1, are further condensed through the elliptical mirror 2 at the second focal point F2 of the elliptical mirror 2. In this way, all of light emitted from the light source 1 and reflected by the elliptical mirror 2 and the reflecting mirror 4 can be condensed at the second focal point F2 of the elliptical mirror 2. Further, as shown in FIG. 3, when the condenser lens 5 is interposed between the elliptical mirror 2 and the light transmitting fiber 3, the rays are uniformly condensed, with a high condensation density, and can be practically brought to a focus. Consequently, it is found that each of the optical illumination devices proposed in the past, when using the lamp which is short in light emission length as the light source and approximates the point source, is capable of condensing most of the rays emitted from the light source 1 in a small area and brings about a desired effect.
Subsequently, another case is considered in which the light source has a relatively long light emission length in the direction of the optical axis. The light source, as shown in FIG. 4, can be regarded as an assembly of point sources. Thus, in view of a condensation principle when the light source is placed on the optical axis, the case is discussed in which the center of the light source having a dimension in the direction of the optical axis is located at the first focal point F1 of the elliptical mirror 2 and, as in FIG. 4, three point sources a, b, and c are provided as the constituents of this light source. Rays emitted from the point source b located at the first focal point F1 are condensed at the second focal point F2 by the elliptical mirror 2. On the other hand, other rays emitted from the point source c shifted from the first focal point F1 toward the light transmitting fiber 3 are condensed below the optical axis, and the remaining rays emitted from the source point a shifted from the first focal point F1 toward the elliptical mirror 2 are condensed above the optical axis, having great distances before and behind the second focal point F2. In this case, an image, projected by the elliptical mirror 2, of the light source having the dimension in the direction of the optical axis is inclined with respect to the optical axis. Moreover, in view of a cross section containing the second focal point F2, the image has a considerably wide condensation circle.
As the position of a point source is distant from the first focal point F1, rays emitted from this point source are condensed farther away from the second focal point F2. Consequently, only rays emitted from point sources situated in close vicinity to the first focal point F1 can be rendered incident on a comparatively fine fiber placed adjacent to the second focal point F1. Furthermore, where there is a peak distribution in the vicinity of the electrodes of ends (which corresponds to a region far distant from the first focal point F1) constituting the light source (as in the metal halide lamp), it is found that the condensation efficiency is entirely impaired.
Thus, when the light source has a relatively great light emission length in the direction of the optical axis, as shown in FIG. 5, the case is discussed in which the reflecting mirror 4 having the center of curvature at the first focal point F1 of the elliptical mirror 2 is placed opposite to the elliptical mirror 2, as proposed in the past. It is assumed that the light source is placed so that its center coincides with the first focal point F1 of the elliptical mirror 2. In the light source having the dimension in the direction of the optical axis, as mentioned above, only the rays emanating from the vicinity of the first focal point F1 of the elliptical mirror 2, after being reflected by the elliptical mirror 2, can effectively enter the light transmitting fiber 3. On the other hand, in the case of rays diffused between the opening edge 2a of the elliptical mirror 2 and the entrance end 3a of the light transmitting fiber 3, the rays originating from the points a and c are collected at the points c and a, respectively, by the reflecting mirror 4. In this way, rays to be collected in the vicinity of the first focal point F1 by the reflecting mirror 4 are limited to those emanating from the points, located close to the first focal point F1, of the light source. As a result, the problem is encountered that, even with the reflecting mirror 4, only the rays emanating from the points, close to the first focal point F1, of the light source having the dimension in the direction of the optical axis are condensed in a small area.
When the light emission length of the light source is long, as depicted in FIG. 6, the case is discussed in which the condenser lens 5 is interposed between the elliptical mirror 2 and the light transmitting fiber 3, as proposed in the past, to demagnify the projected image of the light source and increase the amount of light incident on the light transmitting fiber 3. Since, as stated above, the projected image of the light source formed by the elliptical mirror 2 is inclined with respect to the optical axis, it is impossible that the rays are uniformly collected by condenser lens 5 and the projected image of the light source is demagnified. As such, rays capable of being incident on the light transmitting fiber 3 are eventually limited to those emanating from the points, close to the first focal point F1, of the light source, and hence there is the problem that the placement of the condenser lens 5 brings about very little effect.
In the optical illumination device for introducing the light from the light source into the light transmitting fiber 3, where the metal halide lamp or halogen lamp which has a long light emission length is used as the light source, the luminance itself of the light source is low, and thus rays from respective regions of the light source must be condensed in a small area, without loss. If, however, the light source having the long light emission length is used in the conventional optical illumination device, as stated above, only the rays emanating from the points, close to the first focal point F1 of the elliptical mirror 2, of the light source can be condensed in the small area, and a large amount of light will not be introduced into the light transmitting fiber 3. In order to effectively use the light source having the long light emission length, the optical illumination device with a shape and construction which are most suitable for the light source is required.